Turbo machines are subjected to thrust loads, both radial and axial. It is necessary for the bearings of the machine to support such loads but excessive loads cause power loss and wear. Accordingly, means are needed to control shaft loads. This invention pertains to controlling the axial thrust load of an overhung steam turbine.
By nature of its design an overhung turbo machine has an axial thrust load imposed upon it generally equal to the product of: (1) the area of the missing shaft protuberance (i.e., overhung end); and (2) the differential pressure relative ambient applied to this area. This invention counteracts this problem by employing a sealed balance piston on the overhung shaft end which can be vented to an ambient to balance the thrust or vented to another control pressure to counteract any other net unbalanced force across the turbine.
Conventional methods of solving this problem have included the use of thrust bearings. The use of thrust bearings in such applications is shown in U.S. Pat. Nos. 4,304,522 to Newland; 4,241,958 to Moller et al.; 4,005,572 to Giffhorn; 3,941,437 to MacInnes et al.; 3,817,568 to Elwell; 3,702,719 to Hoffman; and 2,877,945 to Trebilcock. Thrust bearings work best, however, when the load is relatively constant or slowly varying over time.
In the case of gas turbines, positive pressure has been used to help equalize the pressure differential across the rotor shaft. An approach using exhaust air or gas is illustrated in U.S. Pat. Nos. 3,565,543 to Mrazek and 4,152,092 to Swearingen. Still another approach has employed labyrinth seals to prevent leakage from a high pressure to a low pressure region to stabilize the loading on the thrust bearings. U.S. Pat. Nos. 3,129,922 to Rosenthal and 3,043,560 to Varadi are two basic examples. Labyrinth seals have also been used on the downstream side of a rotor to inject cooling air into a chamber or region downstream of the rotor vanes (i.e., U.S. Pat. No. 3,527,053 to Horn and 3,989,410 to Ferrari). However, in overhung steam turbines, such as that shown in U.S. Pat. No. 2,795,371 to Buchi, Sr., et al., these techniques have not been perfected to the satisfaction of the industry.
Even with such pressure equalizing features to minimize axial thrust variations, thrust bearings still wear out and must be replaced. In some overhung turbines, the thrust and journal bearings are enclosed in a bearing housing located immediately upstream of both the rotor and an associated shaft seal assembly. This arrangement improves rotor stability. In some compact designs both the thrust bearings and the journal bearings are often mounted in a common housing or bearing case which must be designed to allow removal of the rotor shaft through the turbine casing. This is by far a difficult and time consuming task. Thus, any solution to the problem of varying thrust loading must accomodate the practical necessity of both shaft removal and bearing replacement under difficult conditions of access.